Japanese Patent Laid-Open No. 2006-197276 discloses a simultaneous-ON prevention circuit that prevents the switching ON of a plurality of relays at the same time.
The simultaneous-ON prevention circuit includes a first transistor connected to a first semiconductor relay via a first diode and controlled by a controller. The circuit also includes a second transistor connected to a second semiconductor relay via a second diode and controlled by the controller. A first resistor is interposed between the first semiconductor relay and the power supply, and a second resistor is interposed between the second semiconductor relay and the power supply. The simultaneous-ON prevention circuit includes a third diode connected at two positions, that is, at a position between the second resistor and the second semiconductor relay and at a position between the first diode and the first transistor. The circuit further includes fourth diode connected at two positions, that is, at a position between the first resistor and the first semiconductor relay and at a position between the second diode and the second transistor.
In such manner, the simultaneous-ON prevention circuit is configured to so ground the upstream portion(s) of the semiconductor relay(s) using the third diode and/or the fourth diode even when both transistors are simultaneously turned ON, for example, due to a controller malfunction. Therefore, the simultaneous-ON prevention circuit keeps an OFF state of the semiconductor relay, and prevents a simultaneous ON of the plurality of relays.
However, the simultaneous-ON prevention circuit includes the third diode connected to an upstream portion of the second semiconductor relay and the fourth diode connected to an upstream portion of the first semiconductor relay. Therefore, the simultaneous-ON prevention circuit needs to have additional ports to connect external connections to the upstream portions of each of the semiconductor relays to the third and fourth diodes, respectively, as well as having to have an increased number of wire harnesses.